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MAPPING THE MOHO IN SOUTHERN CALIFORNIA USING P RECEIVER FUNCTIONS
by
Panxu Zhang
A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(GEOLOGICAL SCIENCES)
December 2012
Copyright 2012 Panxu Zhang

The degree to which faults are localized or distributed within the continental lithosphere has long been a controversial subject. This thesis presents a study of the variation for the crustal thickness in southern California. The goal is to study strain deformation at depth by investigating the variations of the Moho beneath strike-slip faults. The data used in this study are broadband teleseismic waveforms from 2000 to 2011 recorded by the Southern California Seismic Network (SCSN) and USArray. The P Receiver Function (RF) method is used to process the teleseismic events to image the Moho. Synthetic modeling in 3D elastic media using a finite difference algorithm is conducted to constrain the geometry of the Moho. ❧ The first part of this thesis presents a map of Moho depth in southern California. The estimated average Moho depth is 30 km but has a range between 18 and 41 km. A shallow Moho of 18-20 km is observed in the Salton Trough and the Inner Continental Borderland. There is a general correlation of a deeper Moho beneath mountains, such as the Peninsular Range, eastern Transverse Ranges and western Transverse Ranges. The deeper Moho beneath these areas is consistent with the presence of the mountain root. Moreover, using a similar broadband seismic data and P receiver function technique as Yan and Clayton (2007), our receiver gather analyses confirm the previous conclusion of a vertical Moho offset beneath the San Gabriel Mountains. ❧ The second part of the thesis involves a detailed study of the Moho beneath the San Jacinto fault zone. First, receiver gathers as a function of back azimuth were analyzed. Receiver gathers at certain stations near the San Jacinto fault trace show a strong back azimuthal variation. These back-azimuthal variations of the Moho signal indicate three-dimensional complexity beneath the central San Jacinto fault that may suggest variations of Moho depth. A SW-NE stacking profile across the Elsinore, San Jacinto, and San Andreas faults indicates an 8~10 km vertical offset structure beneath the San Jacinto fault. In order to constrain the geometry of the Moho in this area, 3D synthetic modeling using finite difference algorithm was conducted to confirm the interpretation of the Moho offset. Six possible geometries were constructed: one with a 10 km vertical step in the Moho, one with a 5-km-wide and 10-km vertically offset ramp, one with a 15-km-wide and 10-km vertically offset ramp, one with a flat Moho but with 10% velocity contrast across the fault, and one with a flat Moho but with 20% velocity contrast across the fault. The synthetic receiver gathers were plotted and analyzed both as a function of back azimuth and incident angles. Our basic conclusion is that the Moho model with a vertical step of 10 km best fits the data. ❧ Our results support the idea that the lower crust is strongly coupled to and deforms with the upper crust. For the deformation observed beneath the strike-slip faults, our back-azimuth RF analysis and synthetic modeling found a 10-km Moho step beneath the San Jacinto fault. This result suggests that the fault extends through the entire crust and that the strain in the lower crust is localized within a narrow zone beneath this major strike-slip fault.

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MAPPING THE MOHO IN SOUTHERN CALIFORNIA USING P RECEIVER FUNCTIONS
by
Panxu Zhang
A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(GEOLOGICAL SCIENCES)
December 2012
Copyright 2012 Panxu Zhang